This invention refers to insulating or protective coverings, particularly to shed-formed insulating or protective coverings for electrical devices, such as composite insulators, voltage dischargers and terminals for electrical cables, and the manufacturing method thereof.
Outdoor terminals are known, for example, for connecting a cable to an overhead electrical line, usually comprising an insulating covering provided with means for connection to a support pylon, housing within it the terminal portion of a cable whose conductor extends to the extremity of the insulating covering to be connected to the overhead line.
According to the known art, the insulating covering with the function of ensuring insulation between the end portion of the live cable conductor and the earthed supporting structure of the terminal or base consists of an element with a surface extension sufficient to restrain the passage of current along the outer surface of the terminal. This element usually has sheds, according to need, that by increasing the surface area increase the leakage path between the upper end of the terminal and its base so that it can resist surface discharges.
In a known form of embodiment from "New Prefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cables" (Underground Transmission and Distribution Conference, 1974, pages 224-232), an outdoor terminal comprises, in particular, a base plate to which is secured the base of a shed-formed body made of porcelain, to the upper end of which the cable conductor is connected through appropriate means of support and connection; an earth electrode and a field control cone, of polymeric material, is forced onto the cable insulating surface within a cylinder of epoxy resin where it enters the shed-formed body, while the free space within the shed-formed body is filled with insulating oil.
The purpose of said insulating oil within the porcelain covering is to eliminate air, subject to possible ionization where the electrical field is highest, with consequent impairment of the terminal integrity.
The necessity has arisen, e.g. in the covering of high-voltage outdoor terminals for cables with extruded insulation, to replace the porcelain with polymer and composite materials for various reasons, including:
reduce to the minimum the risks of explosion in case of fire or internal electrical discharge; PA1 reduce the weight; PA1 reduce the brittleness, so as to prevent damages caused by accidental impacts or, for example, by vandalism; PA1 increase the simplicity and ease of transport and installation; PA1 increase the safety margins under conditions of heavy pollution. PA1 in high-voltage outdoor terminals, for cable with extruded insulation, designated "dry terminals" because they are devoid of both the porcelain covering and the insulating oil, such as those described in the article "Extremites synthetiques: vers la tres haute tension", by P. Argout, M. Luton, presented at Jicable 1995; PA1 in self-supporting dry terminals, as for example in European patent application no. 95106910.3; PA1 in voltage dischargers for medium and high voltage and similar.
The replacement of porcelain in the terminal, as is known for example from CIGRE' 1992, 21-201, entitled "Synthetic Terminations for High Voltage Cables--Assessment of Service Life", is achieved by using tubes (cylindrical and/or conical) of glass-packed resin covered with anti-tracking insulating rubber, which must protect the underlying part against the penetration of moisture and furnish the necessary leakage path (shed-formed profile) to surface current.
Tracking, as defined in IEC standard 1109 of 1992, is an irreversible surface degrading of insulating material with the formation of conductive paths even under dry conditions.
Coverings of anti-tracking insulating rubber are also used, for example,:
For the same reasons glass and porcelain insulators (for supporting the conductors of overhead lines, for example) are also increasingly replaced with so-called "composite insulators", consisting essentially of bars of glass-packed resin which are also covered with anti-tracking insulating rubber with shed-formed profiles.
Some of the types of rubber most commonly used for these anti-tracking coverings with shed-formed profiles are, for example, compounds with an EPR or silicon rubber base.
For the purposes of this description, EPR shall be taken to mean compounds with ethylene-propylene polymer bases, which include in particular the compounds based on EPM (ethylene-propylene copolymer) or EPDM (ethylene-propylene-diene terpolymer).
Various procedures are known for producing these shed-formed rubber coverings and, where required, ensuring their adherence to the element being covered (e.g. a tube of glass-packed resin for the terminals of a cable or a glass-packed resin bar for composite insulators).
One of the most common methods is molding, typically compression molding, as described, for example, in the European patent applications published with number 120,187 or 71,953, or injection molding, as described in American U.S. Pat. No. 4,670,973 or in Japanese patent application no. 92JP 1763.
The drawbacks of the known techniques for manufacturing shed-formed coverings by molding are caused primarily by the need to use increasingly complex and costly molds as the dimensions of the covering increase and by the production difficulties linked to the increase in molded volume. Other difficulties derive from the need to produce small lots or just prototypes with a given mold. In fact, the mold is an element linked with highly precise shapes and dimensions, and given the complexity of the mold cavities it cannot be easily modified; thus the mold is hardly adaptable to the different needs of the users. The leakage path required for a particular product, for example, varies according to the atmospheric pollution of the area of installation, and this may require the presence of a different number of sheds for a given product and a given profile, hence the need for different molds to produce the number of sheds required. In other cases, the shed profile may vary to satisfy specific user needs, necessitating, as above, the use of a new mold capable of molding the new profile requested.
In addition, as the service voltage increases (60 kV, 90 kV, 150 kV, 245 kV, 400 kV) so the dimensions of the elements to be covered increase and thus the coverings themselves. In the case of high voltage terminals, for example, for voltages of 150 kV the covering diameter is usually between 300 and 350 mm and long around 1500 mm; for voltages of 245 kV, the diameter is usually between 400 and 450 mm, while the length is between 2500 and 3000 mm; for voltages of 400 kV the diameter is usually between 500 and 600 mm and the length between 4000 and 5000 mm.
It is clear that with these dimensions it becomes rather difficult to produce the covering by molding it in a single cycle. The molding should thus be executed in a number of operations, causing further technical problems.
Furthermore, the use of injection-molding technologies limits the selection of polymer material basically to compounds with moderate or low viscosity which can be easily injected into the molds but which often present a compromise regarding the resistance to the tracking effect. From this standpoint, for example, the compounds with liquid silicon rubber bases are the easiest to inject but offer only modest resistance to the tracking effect and particularly high costs.
In cases where compression molding technology is used, it is possible to utilize a broader selection of polymer materials having greater viscosity under molding conditions than the previous types, on the order of 50-70 Mooney at 100.degree. C., for example. Nevertheless, even in this case there are growing difficulties with the size of the mold. In particular, the quantity of material necessary to fill the mold impression must be metered carefully. It is also especially difficult to distribute the material uniformly within the mold impression, given its reduced fluidity, especially when the sheds are large (the pieces may reach 600 mm in diameter). Since the material is unable to adapt to the precise shape of the mold impression, the resulting covering will probably have numerous surface imperfections, which can reduce its resistance to the tracking phenomenon.
Another problem is caused by the residual material left from previous moldings. In fact, the deeper and narrower the mold cavities, the more likely it is that small portions of material will remain attached inside them when the piece is extracted, causing imperfections in the sheds molded thereafter. In particular, it is possible that after a few moldings (four or five) the process must be interrupted to clean the mold impression from accumulated residues, with obvious damage due to the reduced productivity of the machinery.
This is followed by further difficulties in extracting the covering from the mold once it has been vulcanized. Particularly the sheds, and their edges which are is thinner, may stick to the mold, making the extraction operation difficult, in some cases, and even causing it to be torn during the mold opening step, for example. The damage may require the piece to be reworked to repair it, but in some more serious cases the piece may be completely discarded, in some cases along with its support (e.g. the tube or bar of glass-packed resin), greatly increasing production costs.
To obviate some of these problems, therefore, technologies alternative to the molding of monolithic units with shed-formed profiles have been sought. The high costs of the raw material, particularly in the case of silicon rubber, have resulted in a search for processes that could also resolve the problem of minimizing scrap material, which in this highly competitive sector may have a considerable impact on the costs of the finished product.
The French patent application published with no. 2 579 005 describes a process in which initially the individual sheds are molded, then vulcanized and then forced onto a bar of glass-packed resin covered with raw rubber and the entire piece vulcanized in an autoclave. This method, however, though on the one hand it avoids the molding of monolithic units, on the other requires the use of a series of molds capable of forming the various sizes of sheds. Then the sheds produced must be forced one by one onto the support, generating gaps between sheds in which anomalous contaminants (such as salts or powders) could deposit, causing concentrated and accelerated electrical erosion.
The Japanese patent published with no. 6139860 calls for the covering to be extruded onto the support and subsequently shaped with sheds by means of suitable blocks that wrap around it while they transmit movement to it. But in this case it is difficult to achieve proper vulcanization without modifying the specified profile. Furthermore it is easy to cause surface irregularities in the area near the various blocks, with the well-known problems they cause and the consequent need to eliminate them with successive operations.
In known alternative techniques, a suitable profile is extruded and wrapped in spiral fashion on its support (tubes or bars of glass-packed resin) previously covered with rubber, and then the assembly is vulcanized, as described, for example, in the French patent published with no. 2,363,170, in the Canadian patent application published with no. 2,046,682 and in Swiss patent no. 640,666.
Again in this case, however, there are several drawbacks. First, it is not possible to obtain all the types of profiles, some of them particularly advantageous, in a sufficiently practical manner. In particular, one of the profiles best suited for use in highly polluted areas is the one with alternating large- and small-diameter sheds. In this case it would be necessary to extrude a profile with a variable diameter, which is difficult to achieve. In addition, the new profiles, since they are for helical construction, do not ensure the absence of continuous water paths in the presence of rain or heavy humidity, increasing the risk of surface discharges. It is even more complex to ensure precise respect of the angles indicated in the design of these sheds, since during extrusion the raw rubber mixture tend to vary their inclination with respect to the longitudinal axis, due to the effect of gravity, temperature and the time necessary to reach a sufficient degree of reticulation to fix their position. Lastly, the extrusion of helical profiles, with overlapping edges, inevitably generates gaps that must be refinished later to avoid abnormal deposits of contaminants (e.g. salts or powders) that are a cause of concentrated, accelerated electrical erosion.
The French patent application published with no. 2,523,361 describes another procedure for producing a spirally shed-formed profile. Initially, a layer of polymer is extruded onto a support, then this layer is modeled by passing a spirally wrapped cable onto it, so as to create a spirally shed-formed profile. It is important to note that the removal of material caused by the passage of the cable on the sleeve causes a dangerous effect: in fact, the dragging of the material by the cable could result in a profile that is irregular and thus more exposed to the effects of tracking. Added to this defect are the well-known disadvantages of spiral profiles mentioned above.